I actually just did this. Typically, I'd do 3 gallons boiled the night before brew day. I am extract brewing, partial boil. Anyway, 3 gallons night before is boiled and left to cool. Brew day, I'd typically take 2.5 gallons into the brew kettle right from the tap. However, this last one I used exclusively Brita filtered water. I, too, would like to know if this is making a difference to the result, positive or negative.